Each item in your library can be tagged with keywords, annotated and linked to other related articles in your Zotero Library. These functions appear next to the item info tab in the right hand corner of your library:
Notes: This tab allows you to annotate your records with a note that you have written yourself. A note might, for example, be a brief explanation to yourself about why the article is useful to your study, or how it is related to another article.
Tags: This tab allows you to create your own keyword tags for items in your Zotero library. These can be any words you choose, but maintaining consistency in the words you use is important. This function allows you to search your library for that keyword, to get a list of items that you have tagged. You will also notice that many items that you save in your library will already have tags. These tags are an automatic part of the import process and can be used or deleted as you please. The tags also appear at the bottom of the Collections column (bottom left hand of your library).
Related: This tab allows you to build up a list of other items in your library that are related to that particular item.